Controlling nitrogen content of hydrocarbon oils



Jan. 19, 1965 P. D. HARVEY 3,166,493

CONTROLLING NITROGEN CONTENT OF HYDROCARBON OILS Filed May 14, 1962 CATALYST CONTAMINATED CATALYST-FREE v OIL 3 OIL OIL I I I4 4 CONTACTING SEPARATION RECOVERY ZONE ZONE ZONE 2/ I 15 1 17/ I I 1.:

OIL-FREE 10/ CATALYST V CATALYST-OIL SLURRY INVENTOR PHILLIP D. HARVEY United States Patent Ofifice 3,166,493 Patented Jan. 19, 1965 3,166,493 CQNTROLLHIG NlTRQGEN CQNTENT @F HYDRUCARBQN @ELS Phillip l). Harvey, Walnut Creek, (Ialih, assignor to California Research (Jorporation, San Francisco, Calif., a corporation of Delaware Filed May 14, 1962, der. No. 194,253 5 Claims. (Ql. ass-cs4) This invention relates to processes for the removal of contaminating nitrogen compounds from hydrocarbon oils. More particularly, the invention relates to processes for reducing the nitrogen content of an oil by contacting the oil with an adsorbent.

The removal of contaminating nitrogen compounds from a hydrocarbon oil is known to improve the properties of the oil in many respects. For example, the color is usually improved, thereby enhancing the market value of a salable oil. Also, the oil is made more amenable to further processing, such as catalytic cracking, reforming, hydrocracking, and hydrogenation. It has been found that control of the nitrogen content at a fixed, stable, low value is especially important in certain embodiments of processes like catalytic hydrocracking and catalytic reforming.

It is known that nitrogen compounds can be removed from oils by adsorption on certain solids. Heretofore, however, adsorption has been successfully used only in systems where a relatively fixed percentage or a fixed amount of the initial nitrogen content of the oil is to be removed, not to control the nitrogen content. For example, a portion of the nitrogen compounds may be removed from a catalytic cracker feed by contacting the oil with a portion of the cracking catalyst prior to contacting for cracking with the bulk of the catalyst. In such processes, no provision is made for controlling the nitrogen content of oil to any particular value, as the system is adapted only to effect a directional improvement in the properties of the oil by removing some fraction of the nitrogen compounds, as determined by the ratio of catalyst used for adsorbing to the oil feed. Also, adsorbents have been used in fixed-bed contacting systems wherein an oil containing nitrogen compounds is passed through a bed of adsorbent until the adsorbent is essentially saturated. In such systems the nitrogen compounds are nearly completely removed from the first portion of oil passed through the bed, but the nitrogen content of the contacted oil gradually or rapidly increases as more oil is passed through the bed. A cyclic process may be provided wherein contacting of the oil with adsorbent in one bed is terminated when the nitrogen content of the contacted oil reaches some particular value, and the oil is then passed through a fresh or regenerated bed of adsorbent while the first bed is recharged or regenerated. Operating costs of such cyclic processes have generally been excessive and adsorbent life has been unsatisfactorily short.

An object of this invention is to adjust the nitrogen content of an oil to a relatively fixed concentration.

Another object is to control the nitrogen content of a hydrocarbon oil to be converted in a catalytic conversion unit using a nitrogen-sensitive catalyst, by adsorbing nitrogen compounds on finely divided crackingcatalyst in a slurry system in such a way that excess adsorption capacity is always available to compensate for increases in the nitrogen content of the oil from which nitrogen compounds are to be removed.

In accordance with the invention, nitrogen compounds are removed from a hydrocarbon oil to adjust or control the nitrogen content thereof by passing a liquid hydrocarbon oil feed containing nitrogen compounds to a contacting zone, adding finely divided cracking catalyst to said contacting zone to form therein an oil-catalyst slurry at conditions at which a substantial portion of the nitrogen compounds adsorb on the catalyst, maintaining a higher ratio of catalyst to oil in said slurry than is represented by the ratio of added catalyst to oil feed by passing a portion of the slurry to a separation zone and therein separating catalyst-free oil reduced in nitrogen content from a denser slurry of catalyst and oil, at least a portion of which denser slurry is returned to said contacting zone, recovering said catalyst-free oil, and withdrawing catalyst having nitrogen compounds adsorbed thereon from the system.

By proceeding in this way, the nitrogen content of the catalyst-free oil can be adjusted or controlled to a desired concentration, within relatively narrow limits, by adjusting the rate of addition of cracking catalyst and the rate of withdrawal of catalyst having nitrogen compounds adsorbed thereon, to compensate for changes in the nitrogen content of the hydrocarbon oil feed to the contacting zone. Excess adsorption capacity is maintained in the system at all times, so that the adjustment in the rate of addition of cracking catalyst and/0r rate or withdrawal of catalyst having nitrogen compounds adsorbed thereon need not be made immediately or every time that a change in the nitrogen content of the oil feed occurs. At any instant, the rates of catalyst addition and withdrawal need not be equal, as the inventory in the slurry may be temporarily built up or depleted.

The cracking catalyst added to the contacting zone should be in the form of particles small enough to form a slurry in oil. For example, it may be all or a portion of the fresh catalyst makeup to a fluid-bed catalytic cracking process, or it may be a minor portion of regenerated catalyst withdrawn from the fluid-bed catalytic cracking unit. A synthetic silica-alumina, acidactivated clay, silicamagnesia composite, or the like may be used. Catalyst having nitrogen compounds adsorbed thereon'may be' free catalyst is then passed to the catalytic cracking process. In this way, the nitrogen removalis very economically accomplished because a continuous non-cyclic process can be used, and there is no need to provide separate facilities for adsorbent elution or regeneration to remove the adsorbed nitrogen compounds. Since only a small portion of the inventory of cracking catalyst is involved, the operation of the catalytic cracking process is not significantly affected. The cracking catalyst is known to be very rugged, and it is a good adsorbent for nitrogen compounds.

The attached drawing is a block flow diagram illustrating the basic features of the invention and certain preferred embodiments thereof.

Referring to the diagram, a contaminated oil, com-' prising a liquid hydrocarbon oil containing nitrogen compounds, is passed via line 1 to contacting zone 2; Cracking catalyst is also added to contacting zone 2 through line 3. As mentioned, the cracking catalyst may be fresh makeup catalyst or a portion of regenerated catalyst. In either event, it should be substantially free of coke in order that the adsorbtive sites will be available for adsorbing nitrogen compounds. A slurry is formed in contacting zone 2, maintained by suitable mixing or stirring devices if desired, at conditions at which a substantial portion of the nitrogen compounds initially contained in the liquid hydrocarbon oil adsorb on the cracking catalyst. Said conditions must include a temperature low enough such that the hydrocarbon oil remains in the liquid phase and so that cracking or other conversion reactions do not occur to any significant extent. Thus, the temperature should be below about 600 F, or lower if the oil is lower boiling, and preferably below about 300 F. for more efiicient adsorption. Although the adsorption equilibria are most favorable at the lowest temperatures, it is best to maintain the temperature at about 100 F. or higher to improve the fluidity of the oil and the rate of diffusion of nitrogen compounds into the pores of the catalyst where they can attach to absorbtive sites.

In accordance with the invention, there is maintained in contacting zone 2 a catalyst-to-oil ratio higher than the ratio of catalyst added in line 3 to oil introduced through line 1. This is accomplished by continuously withdrawing slurry from zone 2 and passing it through line 4 to separation zone 5. In separation zone 5 the slurry is separated into catalyst-free oil reduced in nitro gen content, recovered through line 6, and a denser catalyst-oil slurry withdrawn through line 7. At least a portion of this denser slurry is returned to contacting zone 2 through line 8. In separation zone 5 the separation may be accomplished in any of several known ways, including filtration, simple settling in a vessel or pond, or under the influence of centrifugal force. erably, the latter means is employed, for example, in a cyclone or hydrocyclone for separating liquids from solids. In the nature of things, it is diificult to obtain in a simple separation both dry solids and solid-free liquids. In accordance with the invention, the separation means used is so constructed, operated, and controlled as to provide an essentially catalyst-free liquid hydrocarbon oil in line 6 while permitting some oil to be entrained with the catalyst in line 7.

Catalyst having nitrogen compounds adsorbed thereon is to be withdrawn from the system. In the preferred method, this is accomplished by withdrawing a portion of the denser slurry of line 7 through line 9. Alternately, a portion of the slurry in contacting zone 2 may be withdrawn, as by diverting a portion of stream 4 through line 15. In either case, the slurry of oil and catalyst carrying adsorbed nitrogen compounds may be withdrawn directly from the system through line 10. More advantageously, the withdrawn slurry is passed through line 11 to recovery zone 12.

In recovery zone 12 the entrained oil is separated from the catalyst, and absorbed non-nitrogenous hydrocarbons are stripped from the catalyst to provide an essentially oil-free catalyst having nitrogen compounds adsorbed thereon, which is withdrawn through line 13. Hydrocarbon oil reduced in nitrogen content, separated and/or stripped from the catalyst, is withdrawn through line 14. For example, in recovery zone 12, the catalyst may be stripped counter-currently with steam, to carry the oil and steam overhead, and the oil may then be separated from the steam by cooling to condense out the water. To provide better cleanup of the catalyst, or where a relatively viscous oil is involved, other means may be employed for recovering the oil from the catalyst. For example, a lighter, less viscous oil, such as a pentane fraction, may be used to displace the entrained and adsorbed hydrocarbons from the catalyst. The pentane may then be separated from the oil by simple distillation, and the oil recovered through line 14 while the pentane is recycled continuously. The pentane adsorbed on the catalyst may be readily removed by means such as the aforementioned steam stripping, and the pentane recovered for reuse. The treating means used in recovery zone 12 do not remove the tightly bound adsorbed nitrogen compounds. Thus the oil-free catalyst withdrawn through line 13 still has the nitrogen compounds adsorbed thereon. This catalyst is then passed to a catalytic cracking process for utilization therein.

Pref- Superatmospheric pressure may be maintained in zones 2, 5, and 12 by means of an atmosphere of gas not reactive with the oil at the conditions used. For example, nitrogen or hydrogen may be used. This is advantageous when the oil is to be further treated in a high pressure conversion process, particularly when the oil is pretreated in another high pressure process, to avoid having to pump the oil repeatedly from atmospheric to process pressures.

In the operation of the process as described above, oil is continuous y passed through line to separation zone 5; purified oil is continuously recovered through line 6; and a denser slurry of catalyst and oil is continuously returned to contacting zone 2 through lines '7 and 8. With respect to the addition of catalyst through line 3 and the withdrawal of catalyst-oil slurry through line 10 or catalyst through line 13, the operation may be either continuous or intermittent or both. Where a fairly large amount of nitrogen is to be removed, say 10 p.p.m. or more of nitrogen, based on the oil, the process is best carried out continuously with respect to addition and withdrawal of catalyst. Where, however, only a final cleanup of nitrogen is desired, as to remove less than 10 ppm. based on the oil, only intermittent or periodic addition and withdrawal of catalyst may be practiced, as required to maintain relatively stable the nitrogen content of the oil reduced in nitrogen content, recovered through line 6. In the more preferred case, some catalyst will be added continuously through line 3, and catalyst having adsorbed nitrogen compounds thereon will be continuously withdrawn through line 13, but the rates of addition and withdrawal will periodically and/or intermittently be increased for short times.

Since the adsorption of nitrogen compounds on a catalyst is a relatively slow process, when the nitrogen content of the oil feed is low, the adsorptive capacity of the catalyst in the slurry is used up only slowly. The residence time of the oil in the system will usually be in the neighborhood of one hour, but may be as high as six hours or as low as ten minutes. The relative residence time of catalyst in the system will depend on the ratio of added catalyst in line 3 to oil in line 1, and on the extent to which the ratio is built up in contacting zone 2 by means of the recycled denser slurry in lines 7 and 8. These ratios depend on the amount of nitrogen compounds to be removed from the oil and the extent to which the full adsorbtive capacity of the catalyst can be safely utilized while still providing excess adsorption capacity in the contacting zone. In general, the ratio of catalyst to oil in contacting zone 2 should be less than about 1:1, in order that the slurry can be conveniently pumped. The denser slurry in line 7 may be returned from a hydrocyclone mounted above the contacting zone so that gravity fiow may be employed. The greater the amount of nitrogen to be removed, the more catalyst must be added and withdrawn, and the less excess capacity for adsorption there will be, proportionately. Based on these considerations, and the requirement that there be a buildup of catalyst in the contacting zone, the system is most advantageously used in the treatment of oils having a relatively low nitrogen content of below p.p.m. or, still better, below 10 p.p.m., to reduce the nitrogen content to a lower but controlled and stable nitrogen content. It is not well adapted to the treatment of high nitrogen oils such as cat cracker feeds, which may contain 1000 ppm. nitrogen or more.

The operation of the process may be exemplified with reference to a preferred embodiment, wherein the process is used to maintain relatively stable and below a desired low concentration, the nitrogen content of an oil to be converted in a catalytic conversion unit employing a nitrogen-sensitive catalyst. The catalytic conversion unit may be exemplified by a catalytic reformer utilizing a platinum-alumina catalyst, or a low temperature hydrocracking process employing a nickel sulfide or cobalt sulr fide on silica-alumina catalyst. It is known to operate such units in combination processes comprising pretreating a hydrocarbon oil in a first-stage treating unit,'such as a hydrofining unit, to remove nitrogen contaminants therefrom, and then converting the hydrofined or otherwise pretreated oil in the second-stage or catalytic conversion unit. Thus, in-a hydrofining stage the oil may be contacted with a sulfactive catalyst of supported combinations of cobalt, nickel, molybdenum, or tungsten, with hydrogen at 500850 F. and 200-5000 p.s.i.g. to convert the nitrogen compounds to NHg, which is removed. The nitrogen content of the hydrofined ammonia-free oil tends inherently to increase at least temporarily above a desired low concentration when a change in the operation of the hydrofining unit occurs. 'For example, the change in the operation of the hydrofining unit may be agradual decline in the activity of the catalyst used therein to promote the conversion of the nitrogen compounds to ammonia. As the catalyst becomes less active, the nitrogen content of the hydrofined oil will increase. Ordinarily, this is compensated for by changing the operating conditions, as. by raising the temperature in the hydrofining unit. However, the

a situation must first be discovered by noting that the nitrogen content has increased. Hence, during the time that the nitrogen content is increasing and before corrective action is taken in the hydrofining unit, the nitrogen.- sensitive catalyst employed in the catalytic conversion unit is exposed to a concentration of nitrogen compounds in the oil passed'theretoabove the desired low concentration.

Other changes in the hydrofining unit which will inherently cause an increase in the nitrogen content of the hydrofined oil include temporary upsets, minor equipment failures, a reduction in the availability of hydrogen to the hydrofining unit, changes in the nitrogen content or composition of the feed to the hydrofiner, and similar technical difficulties which alwaysari'se in commercial units despite the most careful desig'ii. Even such factors as a change in the atmospheric conditions from day to night, the cooling water temperature employed in various heat exchangers, and the pressure and heating value of fuel gas employed in furnaces will change the heat balance in the hydrofining unit, thereby changing the average operating'temperatures, and causing the nitrogen content of the hydrofined oil to fluctuate.

Although the increase in the nitrogen content of the ing with hydrogen and a nickel-sulfide on silica-alumina catalyst. This catalyst is gradually deactivated by the nitrogen, and the temperature is gradually increased to compensate for activity loss. A small change in the operation of the hydrofining unit may cause the nitrogen content to increase to 0.3 p.p.m. During the time that the nitrogen content is at this higher value the nitrogensensitive conversion catalyst is being deactivated at a rate much higher than before. Thus, if such a situation is allowed to continue, or occurs repeatedly, the useful life of the catalytic conversion catalyst may be drastically reduced.

In accordance with the preferred embodiment vention, the aforementioned combination process is improved in the following manner: With reference to the drawing, 6000 b.p.d. of gas oil, reduced in nitrogen content to 0.5 p.p.m. by hydrofining, is passed through line 1 to contacting zone 2 at 150 F. Fresh fluidized, silicaalumina cracking catalyst is passed through line 3 into contacting zone 2 in a weight ratio of catalyst to oil of 1:60. A weight ratio of catalyst to oil of about 1:2 is maintained in contacting zone 2 by passing slurry conof intaining 11,900 barrels per day of oil through line 4 to 'sepa ration zone 5. From separation zone 5, comprising one or more hydrocyclones, there is withdrawn through line 6 5,900 barrels per day of oil reduced in nitrogen content to 0.2 p.p.m and essentially free of catalyst. A denser slurry containing 6000 barrels per day of oil at a catalystto-oil ratio of 1: l is withdrawn through line 7. The major portion of this denser slurry, comprising 5,900 barrels per day of oil, is returned through line 8 to contacting zone 2. A smaller portion, comprising barrels per day of oil, is withdrawn through line 9 and passed via line 11 to recovery zone 12. In recovery zone 12, the oil is stripped from the catalyst with steam, which steam is condensed and recovered, and the 100 barrelsper day of oil so obtained, reduced in nitrogen content to 0.2 p.p.m., is recovered through line 14. Catalyst having nitrogen compounds absorbed thereon, and essentially free of oil, is withdrawn through line 13. and passed to either the reaction zone or the regeneration zone of a fluid-bed catalytic cracking process. The holdup volume of the systern, comprising the contacting zone and the separation zone, is such that the average residence time of oil therein is about one hour. The average residence time of catalyst therein is about 60 hours.

When the nitrogen content of the hydrofine oil in line 1 increases to 0.8 p.p.m., this increase is readily soaked up by the excess adsorption capacity of the catalyst in adsorption zone 2. Thus the catalyst-free oil recovered through line 6 will still contain only slightly more than 0.2 p.p.m. nitrogen, and the amount of nitrogen on the catalyst withdrawn through line 13 is only slightly increased. Of course, if the oil continues to be introduced to zone 2 with the higher nitrogen content, ultimately the amount of nitrogen on the catalyst will increase, and the nitrogen content of the oil in line 6 will also increase by a small amount. Consequently, if the situation in the hydrofining unit is not corrected, ultimatelyit will be necessary to increase somewhat the rate at which catalyst is added through line 3. However, the holdup of catalyst in the system is large enough such that the process can be continued without change for about 30 hours before the nitrogen content of the purified oil will increase much above 0.22 p.p.m., thus providing ample time to make any required adjustments.

Obviously, if there is a total failure in the hydrofining unit, the slurry adsorption system will be unable to reduce the nitrogen content from the high initial value of 2,000 p.p.m. to the desired 0.2 p.p.m.- However, the adsorbent in the slurry will, even under those circumstances, remove nearly all of the nitrogen compounds at least for the short time required to shut off the feed to the catalytic conversion unit, whereby the damage to the nitrogen-sensitive catalyst is substantially minimized.

In contrast to the above, in a fixed-bed adsorption system reducing the nitrogen content of hydrofined oil from 0.5 p.p.m. to 0.13 p.p.m., an increase in the feed nitrogen to 0.8 p.p.m. is reflected in an increase in the nitrogen content of the bed efliuent to 0.24 p.p.m. within two hours. Thirty hours later the effluent contains over 0.32 p.p.m. nitrogen. This comparison is based on a l20- hour cycle in the fixed-bed system using the same amount of catalyst adsorbent as passes through the process of this invention in hours. The fixed-bed process makes more complete use of the catalyst adsorbent and gives a lower average product nitrogen content if a long cycle time (low space velocity) is used, provided the feed nitrogen remains constant. If, however, the feed nitrogen increases, the product nitrogen Will increase. This is a distinct disadvantage if, for example, the product is to be converted in a low temperature hydrocracking unit because continued adjustment in the operating conditions of the hydrocracking is required to compensate for such changes. Thus, a particular advantage of the invention is that it can maintain a steady nitrogen content, thereby permitting stabilizing the hydrocracking operating conditions at '2] an optimum conversion level. Because conditions can be optimized, operation with a stable nitrogen content of 0.2 ppm. will be found to be superior to operation with a nitrogen content averaging 0.2 ppm. but varying from, for example, 0.1 ppm. to 0.3 ppm.

Similar considerations apply in catalytic reforming and like processes, wherein it has been found advantageous to adjust other variables such as temperature and sulfur, moisture, and halogen content of the oil depending on its nitrogen content. Thus, the invention will also be useful in controlling the nitrogen content of the oil to be reformed at a stable value, whereby such other variables can also be stabilized.

I claim:

1. A process for adjusting downward to a relatively constant value the nitrogen content of a flowing hydrocarbon oil stream subject to temporary increases in nitrogen content, which comprises continuously passing said stream to a contacting zone, adding finely-divided cracking catalyst free of coke and nitrogen compounds to said contacting zone to form therein an oil-catalyst slurry at conditions at which a substantial portion of the nitrogen compounds in said oil adsorb on said catalyst,

continuously passing a portion of said slurry to a separation zone and therein separating said portion into catalyst-free oil reduced in nitrogen content and a dense slurry of catalyst and oil having a higher catalystzoil ratio than the slurry in said contacting zone,

continuously retwining at least a major portion of said dense slurry directly to said contacting zone, continuously recovering said catalyst-free oil,

separately recovering cracking catalyst having nitrogen compounds adsorbed thereon,

and regulating the rate of recovering cracking catalyst having nitrogen compounds adsorbed thereon and the rate of adding cracking catalyst free of coke and nitrogen compounds to maintain the catalyst in said contacting zone in a partially spent condition providing excess adsorptive capacity for removing nitrogen compounds from said oil.

2. The process of claim,1 wherein cracking catalyst having nitrogen compounds adsorbed thereon which is separately recovered comprises a portion of said dense slurry which is not retured to said contacting zone.

3. The process of claim 1 wherein cracking catalyst having nitrogen compounds adsorbed thereon is separately recovered by passing a. portion of oil-catalyst slurry to a recovery zone and therein stripping hydrocarbon oil reduced in nitrogen content from substantially oil-free catalyst having nitrogen compounds adsorbed thereon.

4. The process of claim 1 wherein said flowing hydrocarbon oil stream is a product hydrofined oil obtained continuously from a hydrofining unit, and the nitrogen content of said hydrofined oil tends inherently to increase at least temporarily above a desired low concentration when a change in the operation of the hydrofining unit occurs.

5. The process of claim 1 wherein said hydrocarbon oil feed contains less than 10 p.p.rn. nitrogen.

Mason et al Sept. 25, 1962 Buningh et al Sept. 25, 1962 UNITED STATESMPATVENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,166,493 January 19 1965 Phillip D. Harvey It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 12, for "absorbtive" read adsorbtive line 48 and column 6, line 17, for "absorbed", each occurrence, read adsorbed column 6, line 25, for "hydrofine" read hydrofined column 7, line 31-, for "retwining" read returning column 8-, line 13, for "retured" read returned Signed and sealed this 151: day of June 1965.

(SEAL) Attest:

ERNEST W. SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A PROCESS FOR ADJUSTING DOWNWARD TO A RELATIVELY CONSTANT VALUE THE NITROGEN CONTENT OF A FLOWING HYDROCARBON OIL STREAM SUBJECT TO TEMPORARY INCREASES IN NITROGEN CONTENT, WHICH COMPRISES CONTINUOUSLY PASSING SAID STREAM TO A CONTACTING ZONE, ADDING FINELY-DIVIDED CRACKING CATALYST FREE OF COKE AND NITROGEN COMPOUNDS TO SAID CONTACTING ZONE TO FORM THEREIN AN OIL-CATALYST SLURRY AT CONDITIONS AT WHICH A SUBSTANTIAL PORTION OF THE NITROGEN COMPOUNDS IN SAID OIL ADSORB ON SAID CATALYST, CONTINUOUSLY PASSING A PORTION OF SAID SLURRY TO A SEPARATION ZONE AND THEREIN SEPARATING SAID PORTION INTO CATALYST-FREE OIL REDUCED IN NITROGEN CONTENT AND A DENSE SLURRY OF CATALYST AND OIL HAVING A HIGHER CATALYST: OIL RATIO THAN THE SLURRY IN SAID CONTACTING ZONE, CONTINUOUSLY RETWINING AT LEAST A MAJOR PORTION OF SAID DENSE SLURRY DIRECTLY TO SAID CONTACTING ZONE, CONTINUOUSLY RECOVERING SAID CATALYST-FREE OIL, SEPARATELY RECOVERING CRACKING CATALYST HAVING NITROGEN COMPOUNDS ADSORBED THEREON, AND REGULATING THE RATE OF RECOVERING CRACKING CATALYST HAVING NITROGEN COMPOUNDS ADSORBED THEREON AND THE RATE OF ADDING CRACKING FREE OF COKE AND NITROGEN COMPOUNDS TO MAINTAIN THE CATALYST IN SAID CONTACTING ZONE IN A PARTIALLY SPENT CONDITION PROVIDING EXCESS ADSORPTIVE CAPACITY FOR REMOVING NITROGEN COMPOUNDS FROM SAID OIL. 